With the strengthening of the environmental regulations in the recent years, there has been a dramatic increase in the construction of combined-cycle power plants of high performance and reliability with reduced exhaust pollution, in contrast to coal or nuclear power generation apparatus.
A combined-cycle power generation apparatus as such consists essentially of a gas turbine, a heat recovery steam generator, and a steam turbine.
In the aforementioned combined-cycle power generation apparatus, for the purpose of increasing system efficiency, the combustion gas of high temperature is first produced by combustion of fossil fuel, which is then used to run the gas turbine for generating electric power. The steam is produced in the heat recovery steam generator by high-temperature combustion gas (exhaust gas) discharged from the gas turbine, and said steam is used to run the steam turbine for secondarily generating electric power.
FIG. 4 is an outlined configuration diagram which illustrates the relevant regions of the gas turbine and the heat recovery steam generator (hereinafter HRSG) in the constituent elements of an ordinary combined-cycle power apparatus. A heat recovery steam generator (19) comprises an inlet duct (4), a transition duct (6), a heat exchange having a finned tube, and a stack (10).
The operation principle herein is as follows: First, as the high-temperature combustion gas which is streamed in from the exhaust duct (3) of the gas turbine (2) passes through the flow correction device (7) installed in the transition duct (6) of the heat recovery steam generator (19), the flow therein is evenly distributed, and the combustion gas is influxed into the first heat transfer surface, i.e. a tube bundle for heat exchange in the high-pressure section.
The combustion gas flowing into the tube bundle for heat exchange transfers heat to the water or steam flowing inside the finned tube, thereby producing steam of high pressure and temperature. Then, the combustion gas in the state of low temperature is discharged through the stack (10) to the outside.
Of the components of said heat recovery steam generator (19), the flow correction device (7) installed at the inlet duct (4) is used for forming a uniform flow distribution at the entrance of the heat exchanger in the high-pressure section. Consequently, the flow correction device (7) plays a very important role in enhancing thermal efficiency of a heat recovery steam generator (19) and in preventing damages to the finned tube.
Meanwhile, for generating additional electric power, the steam of high pressure and temperature generated from the tube bundle is forwarded to the steam turbine (not illustrated).
Generally, the amount and the temperature of the exhaust gas coming out from the gas turbine (2) tend to stay at a certain state of condition (level), depending on the gas turbine model. As such, the amount and the temperature of the steam, which can be produced in the heat recovery steam generator (19), are usually set to constant values.
However, as occasion demands, it is necessary to additionally increase the amount of steam production. For example, in the following circumstances, it is necessary to supply additional energy for increasing the amount of the steam produced in the heat recovery steam generator (19): (a) when the steam produced from the heat recovery steam generator (19) is used not only for generation of electric power but also for heating the interior of the plant, etc. or (b) the demands of a user cannot be met with a simple combination of gas and steam turbines.
For these reasons, a duct burner (8) is typically installed at the entrance of the heat recovery steam generator (19) to heat the exhaust gas discharged from the gas turbine (2). In such cases, the oxygen concentration around the duct burner (8) must be high for smooth combustion. Therefore, the additional combustion air should be provided thereto, in addition to the oxygen in the exhaust gas discharged from the gas turbine.
However, in the conventional combined-cycle power apparatus, the facilities for separately providing combustion air were inadequate. Consequently, it could not properly accommodate the situation where the oxygen had to be additionally supplied thereto.
Here, No. 1 in FIGS. 1 and 4, which has not been explained, is an air inlet, and G stands for a generator, and M a motor.
For reference, a heat recovery steam generator, depending on the use or non-use of the duct burner (8), is classified into a supplemental-fired heat recovery steam generator or an unfired heat recovery steam generator.